Excessive daytime sleepiness (EDS) is one of the most disabling symptoms associated with a number of sleep-wake disorders. Patients with EDS comprise more than 50% of patients at sleep-wake clinics in the U.S. Various causes of EDS now have been isolated and their importance to various sleep-wake disorders is becoming clear. It appears that the most important cause of sleep-wake complaints, especially EDS, is sleep fragmentation. yet little is understood about sleep fragmentation. There also is no information about how to treat sleep fragmentation and, thereby, alleviate the daytime symptomatology and impairment. Clearly, an understanding of sleep fragmentation is a prerequisite to clinically managing it effectively. We will determine what aspects of sleep fragmentation are critical. Is it the number of arousals that is critical, or is it the spacing (rate) of arousals? Does sleep fragmentation produce inefficient sleep, or essentially is it comparable to a total reversal of sleep? These issues have to be resolved before treatment strategies for sleep fragmentation can be pursued. Treatment strategies for sleep fragmentation might include reduction of the arousals by removing the disruptive stimulus or blunting its effects, or by compensating for sleep fragmentation with increased nocturnal sleep or by daytime naps. A rational use of these treatment strategies requires that we understand sleep fragmentation generally and the cause of sleep fragmentation of each patient specifically.